The present invention generally relates to a method for determining the servo track pair position and, optionally, the longitudinal tape position for a tape using dedicated servo formats. The tape includes at least one set of data tracks and dedicated servo tracks wherein adjacent servo tracks are defined by longitudinal lines through the edges of erased or non-recorded tape portions. In addition, the present invention further relates to an apparatus for performing the method.
The dedicated servo format for use on 1/4" tape cartridge streamers relies on prerecorded servo tracks. The servo and data track positions are placed according to the principles described in U.S. Pat. No. 5,008,765. However, no teaching is present in the prior art on trow to implement the physical servo tracks, but only the layout of the center lines for data and servo tracks can be ascertained from this patent. Furthermore, the following documents QIC-91-41 (QIC-1 CF), Revision B: "Common Recording Format Specification" (Oct. 8, 1991) and QIC-91-42 (QIC-10 GB), Revision B: "Serial Recorded Magnetic Tape Cartridge for Information Interchange" (Oct. 10, 1991) define the physical and logical layout of the servo and data tracks for the 10 GByte standard. QIC-3000, Revision A: "Proposed Interchange Standard Serial Recorded Magnetic Tape Cartridge for Information Interchange" (Sep. 24, 1991) defines the layout for the 3 GByte standard for 144 data tracks and 24 servo tracks.
A servo demodulation scheme has been proposed; however, since no track position information encoded into the servo tracks themselves is present, a drive-embedded microcontroller must be provided which at least implicitly cares for following the tracks numbers.
This principle is well-known from hard disk drives with a dedicated servo wherein the microcontroller counts cylinder pulses issued when the servo and data heads are moved radially over the tracks, and therefore, can always provide information on which track the servo head is located. This is described in Mee and Daniel, "Magnetic Recording", Vol. 2: Data. Storage, McGraw-Hill Book Company 1988, pp. 55-57. Since the disc spins at a constant angular velocity and the centers of the cylinders do not move with respect to the servo tracks, except for further known thermal expansions and small mechanical vibrations, reliable results can be obtained. However, this principle may fail when tapes are used for two reasons: namely, the problem of track repeatability and the inherent physical friction between the tape and the magnetic recording head.
In a tape drive using a serpentine 10 GByte recording format, the servo tracks will wander with respect to the tape edge, and the tape itself always moves with respect to the cartridge base plate or the reference plane. As a result, the tape moves with respect to the magnetic recording head when the servo is not enabled. According to the above-mentioned QIC standards, the servo track wander is specified to be within .+-.25.4 micrometers with respect to the lower tape edge. The movement of the tape with respect to the cartridge reference plane is specified as "track repeatability" or "dynamic tape movement" and is, according to the above standards, .+-.12.7 micrometers in one direction. However, if the tape transport stops during running and the drive backspaces without servo control, the track repeatability is specified as 25.4 .mu.m in a worst case. If the tape starts up in the original direction again, the repeatability is 12.7 .mu.m. The situation is typically encountered during a data append operation.
A further specification for the 1/4" cartridge includes the clearance between the tape guides and the tape. With a worst-case wide guide and a worst-case narrow tape, this clearance is specified to be 46 .mu.m for most of the 1/4" cartridges in use today. Furthermore, laboratory measurements based on optical detection of the tape movements show that the tape may slip on the tape guides. Normally, these types of dynamic track movements are within the specification for the cartridge. However, a risk exists that the tape movements may be larger, for example, when the tape is accelerated, negatively and positively, during a wind-rewind operation and again during a data append operation. Abrupt tape movements can, in principle, occur when the tape is located with the center line over the midpoint of a tape guide. The tape may slip quickly to one of the sides from this unstable position. This situation may most likely occur when the tape guides are nearly perpendicular to the cartridge base plate or reference plane. Furthermore, the tape may have been stabilized at one side of the guides leaving a gap of up to a maximum of 46 .mu.m. When data is appended to the tape in another drive located in a vertical position (i.e. within horizontal tape guides), abrupt changes of more than 25.4 .mu.m may take place.
If the dynamic tape movements are within, for example, +20 .mu.m during the tape speed ramp-up and rewind operation before the servo is enabled, the actual servo track wanders at the same time within specifications of, for example, +15 .mu.m during the backspace operation. Additionally, the position of the center line of the actual servo track of the tape will deviate +35 .mu.m from the center of the magnetic recording head, i.e. from the center of the servo head or servo heads. Now, the track pitch on 3 GByte-10 GByte tape formats is defined to be 35 .mu.m, and therefore, the center of the servo head(s) will be located near the center of the nominal position of a neighbor track. If the servo attempts to lock, it will fail due to the wrong polarity of the position feedback signal. A decision on whether the servo is located above or below the desired track is not possible or at least very difficult to reliably determine.
The lack of servo track pair numbering may also cause problems when the track pitch is decreased in future 1/4" tape formats. The error may not be capable of being detected in some cases, and data will unavoidably be overwritten.
Normally, the actual track repeatability of a signal cartridge sample investigated in a laboratory will be much better than its specification. However, cartridges may be mass-produced in millions of units, and during normal use, they may also be subject to temperature and humidity changes as well as tape wear. Under all these conditions, data integrity must be guaranteed. The servo system must always find the correct track number for all cartridges and under all changes in environmental conditions.
In addition, the reliability of the proposed QIC servo format is affected by head-to-tape friction. The read heads used for the servo are the same heads as used for the data signals (in total three channels with 6 read gaps of which either one channel (two read gaps at a time) may be used for the servo). Due to low-frequency noise from the magnetoresistive (MR-) element itself (noise induced in the magnetoresistive element because tape asperities cause varying thermal cooling of the element) and the DC-bias current used for the MR-element, the read channels are AC-coupled. A typical high-pass frequency may be about 50 kHz. Further, the servo signal must be demodulated. When the tape speed changes, the carrier frequency also changes with the same percentage. Depending on the actual servo demodulator technique used (fixed-frequency front-end bandpass filters etc.), building a frequency-tracking demodulator may be more or less expensive. Typically, the demodulator may be built for a narrow range of fixed tape speeds, all with carrier frequencies placed above the AC-coupling high-pass frequency.
When the end of the tape approaches, the 1/4" tape drives in use today prepare for a track shift by stopping the tape and moving the head to the location of the next track set to follow. With the 3 GByte/10 GByte tape drives as specified according to the QIC standard, the servo must be disabled during this operation. Thereafter, the tape accelerated in the opposite direction and the servo system tries to lock on the correct servo track. The servo system cannot be active during the complete ramp-down and ramp-up time intervals since the tape speed is considerably reduced, and the servo carrier frequency is also reduced. The signal-to-noise ratio may begin to be degraded, as well, depending on where the cut-off frequency of the system has been placed. The performance of the servo system will also be degraded due to undersampling, and it must be shut off. This may be due to the fact that all transversal tape motions are not to scale with tape speed. Further, other difficulties resulting from characteristic frequency components found in the transversal tape vibrations result in tape slips occurring even at low tape speed. Therefore, the servo heads may be more or less off track after the ramp-down period.
Due to friction between the tape and the head, the tape may stick to the head when it is moved in a direction perpendicular to the tape. This may be observed on a statistical basis as a combination of variations from cartridge to cartridge, variations in humidity, temperature, tape tension, tape position on the tape guide, tape surface conditions, such as more or less worn tapes, and magnetic head surface conditions. Again, the relative effect on the track position from the tape-to-head friction will be greater when the track pitch decreases.
Therefore, moving the head from one track to another without running the tape at full speed cannot be recommended. Track shift must take place before the tape has been stopped or just after it has been accelerated to full speed again in the opposite direction.
When the tape has been stopped and then started again, the servo firmware must also perform a verification of correct polarity of the feedback signal to check if the heads are located on an even or an odd servo track and compare this polarity with the required value. Further, even if the polarity is correct and the servo can lock, the track position may be in error. If the servo cannot lock, the write operation must be stopped by the control firmware and start to count tracks from a known vertical position either below or above the actual servo band. This re-counting must be performed while the tape is running and will cause a delay in the actual tape drive operation, and the streaming operation cannot be maintained.
Recounting of tracks may be difficult and time consuming for a tape drive servo mechanism due to mechanical and reliability reasons. Due to the serpentine nature of the tape drive in contrast to the cyclical nature of a hard file or hard disk, the preferred method for moving the head is a combination of a stepper motor with a broad operating range and a linear, analog actuator operating over a very limited, narrow range. This results in the lowest overall costs and the greatest resistance to mechanical shocks. However, even when a linear, high speed actuator capable of moving the head over a broad operating range is used in a high-performance, high-speed track seeking servo, a need for high reliability exists.
The problems increase for a future tape format when the track pitch is decreased. If the track repeatability of the cartridge in the position of the servo tracks with respect to the tape edge are not improved considerably, the possibility exists that the servo heads may be located two pitches above or below the desired position. Since no track information is encoded into the servo tracks themselves, recorded data may be overwritten and lost.
Methods for automatic tracking are well known, for instance, in consumer video recorders as described in Mee and Daniel, "Magnetic Recording", Volume 3: "Video Recording", McGraw-Hill Book Company, 1988, pp. 53-54. The four-tone frequency method disclosed therein for analog video recorders consists in a recording of four low-frequency pilot tones, each tone being frequency multiplexed with the video recording signal. In addition, all pilot tones are spaced multiplexed cyclically with four video frames or tracks. The video read head, when slightly off-track, senses its own pilot signal and a portion from the neighbor track pilot signal. The ratio of the neighbor track pilot signal to its own pilot signal determines the deviation from the desired position. The face or polarity to be used for the feedback signal is determined by the actual frequency band of the different signal which follows after analog mixture of the two actual pilot signals are detected. The difference frequency detected determines in which direction the head is to be moved.
For helical scan video and data recorders, several other methods exist as alternatives to the four frequency pilot tone method. Some of these are based on recording special patterns of frequency bursts, typically from 2 to 4 possible frequencies, in the beginning of each track. These bursts are space-multiplexed both in the direction of the tracks and along the direction of the tape, as disclosed in U.S. Pat. Nos. 4,121,264, 4,843,493 and 4,843,495. These methods, however, sample the tracking error only once at the beginning of each video or data track. Another method based on obtaining continuous servo information along the tracks is based on azimuth recording and the measurements of timing differences, as described in U.S. Pat. No. 4,868,692.
It is common to all these methods that the tracks are not numbered and only relative positioning information can be demodulated and used for the error signal input to the servo, e.g. the encoded frequency patterns are part of the positioning measurement system of the tracking servo.